In wireless communications, the intensity of radio signals keeps fluctuating due to a change in transmission status of the radio waves between a transmitter and a receiver. Thus, in the receiver, the SN ratio (signal-noise ratio) or distortion rate fluctuates depending on the intensity of received radio signals. In order to solve this problem, a receiver is equipped with a gain control circuit that automatically conducts gain control. The gain control circuit is configured to amplify a signal to improve the SN ratio when the intensity of a received signal is low, and attenuate a signal to mitigate the distortion of the signal, which occurs in the receiver, when the intensity of a received signal is high. In some cases, the intensity of the received signal greatly changes within a range up to 100 dB, for example, and therefore, the receiver is required to have a gain control range of approximately 100 dB. Because it is not possible to predict the timing of the signal intensity change, the gain control circuit needs to operate in real-time when a change in signal intensity occurs. For this reason, there is a demand for a high-performance gain control circuit that has both the wide gain control range and real-time operation capability.
A gain control circuit using an analog circuit has an advantage of high operation speed, and therefore, it is possible to provide excellent performance in the real-time operation. However, a gain control circuit with an analog circuit has several drawbacks such as noise overlapping the main signal or a difficulty to achieve a wide gain control range because the gain changing circuit itself is susceptible to distortion when the reception intensity is high. To solve those problems, a receiver having an automatic gain control circuit that uses a step attenuator configured to change the gain of the amplifier in a step-like manner is proposed (Japanese Patent Application Laid-open Publication No. 2011-199599, for example).